Patchwork [PR43864] Gimple level duplicate block cleanup.

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Submitter Tom de Vries
Date Aug. 19, 2011, 4:24 p.m.
Message ID <4E4E8E2C.1090809@codesourcery.com>
Download mbox | patch
Permalink /patch/110693/
State New
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Tom de Vries - Aug. 19, 2011, 4:24 p.m.
Hi Richard,

sorry for the delayed response, I was on vacation for a bit.

On 07/22/2011 05:36 PM, Richard Guenther wrote:
> On Sun, Jul 17, 2011 at 8:33 PM, Tom de Vries <vries@codesourcery.com> wrote:
> 
>> Bootstrapped and reg-tested on x86_64.  Ok for trunk (after ARM testing)?
> 
> +static int
> +same_succ_equal (const void *ve1, const void *ve2)
> +{
> ...
> +  if (bitmap_bit_p (e1->bbs, ENTRY_BLOCK)
> +      || bitmap_bit_p (e1->bbs, EXIT_BLOCK)
> +      || bitmap_bit_p (e2->bbs, ENTRY_BLOCK)
> +      || bitmap_bit_p (e2->bbs, EXIT_BLOCK))
> +    return 0;
> 
> that's odd - what are these checks for?
>

Turned out to be dead code, now removed.

> +  if (dump_file)
> +    {
> +      fprintf (dump_file, "initial worklist:\n");
> 
> with dump_flags & TDF_DETAILS
> 
> I'm now at merge_calls and wondering about alias info again.  We are
> probably safe for the per-pointer information because we are not
> operating flow-sensitive for memory and for merging require value-equivalence
> for SSA names.  For calls the same should be true - we are not
> flow- or context-sensitive, and even if we were context-sentitive we
> require equivalent arguments (for memory arguments we should be safe
> because of the non-flow-sensitivity).
> 
> So, did you actually run into problems?  If not then I suggest to remove
> merge_calls completely (and the related changes that it requires).
> 

No, I did not run into actual problems. So, merge_calls and dependencies are now
removed.

> +/* Create or update a vop phi in BB2.  Use VUSE1 arguments for all the
> +   REDIRECTED_EDGES, or if VUSE1 is NULL_TREE, use BB_VOP_AT_EXIT.  If a new
> +   phis is created, use the phi instead of VUSE2 in BB2.  */
> +
> +static void
> +update_vuses (tree vuse1, tree vuse2, basic_block bb2,
> +              VEC (edge,heap) *redirected_edges)
> 
> ...
> 
> +  if (vuse2 == NULL_TREE)
> +    return;
> 
> hm, that's the case when there is no VUSE that is dominated by BB2
> (or is in BB2).  Ok, might happen.
> 
> +             locus1 = gimple_location (SSA_NAME_DEF_STMT (arg));
> +             add_phi_arg (phi, arg, e, locus1);
> 
> I don't think virtual operand PHIs should have locations, just use
> UNKNOWN_LOCATION here.
> 

Done.

> +  locus2 = gimple_location (def_stmt2);
> 
> Likewise.
> 

Done.

> +  /* Create a phi, first with default argument vuse2 for all preds.  */
> +  lhs = make_ssa_name (SSA_NAME_VAR (vuse2), NULL);
> +  VN_INFO_GET (lhs);
> +  phi = create_phi_node (lhs, bb2);
> +  SSA_NAME_DEF_STMT (lhs) = phi;
> +  FOR_EACH_EDGE (e, ei, bb2->preds)
> +    add_phi_arg (phi, vuse2, e, locus2);
> +
> +  /* Now overwrite the arguments associated with the redirected edges with
> +     vuse1.  */
> +  for (i = 0; i < EDGE_COUNT (redirected_edges); ++i)
> +    {
> +      e = VEC_index (edge, redirected_edges, i);
> +      gcc_assert (PHI_ARG_DEF_FROM_EDGE (phi, e));
> 
> No need for this assert.
> 

Removed.

> +      if (vuse1)
> +       arg = vuse1;
> +      else
> +       arg = BB_VOP_AT_EXIT (e->src);
> +      SET_PHI_ARG_DEF (phi, e->dest_idx, arg);
> +      locus1 = gimple_location (SSA_NAME_DEF_STMT (arg));
> 
> See above.

Done.

> 
> +      gimple_phi_arg_set_location (phi, e->dest_idx, locus1);
> +    }
> 
> 
> Can you maybe merge this with the update-existing-phi-case?  They
> look all too similar.
> 

Indeed. Merged now.

> +  /* Replace uses of vuse2 in bb2 with phi.  */
> +  FOR_EACH_IMM_USE_STMT (stmt, iter, vuse2)
> +    {
> +      if (gimple_code (stmt) == GIMPLE_PHI)
> 
> Does FOR_EACH_IMM_USE_ON_STMT really not work for PHIs?
> Other code doesn't seem to care.

Now rewritten to use FOR_EACH_IMM_USE_ON_STMT also for PHIs.

> +       {
> +         edge e;
> +         if (stmt == phi)
> +           continue;
> +         e = find_edge (bb2, gimple_bb (stmt));
> +         if (e == NULL)
> +           continue;
> +         use_p = PHI_ARG_DEF_PTR_FROM_EDGE (stmt, e);
> +         SET_USE (use_p, lhs);
> +         update_stmt (stmt);
> +       }
> +      else if (gimple_bb (stmt) == bb2)
> 
> That check looks odd.  A use can very well appear in a forwarder block.
> 

AFAIU, it can't. The pass does not allow forwarder blocks between bb2 and it's
successor(s).

> +       {
> +         FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
> +           SET_USE (use_p, lhs);
> +         update_stmt (stmt);
> +       }
> +    }
> 
> +/* Scans the vdefs and vuses of the insn of BB, and returns the vop at entry in
> +   VOP_AT_ENTRY, and the vop at exit in VOP_AT_EXIT.  */
> +
> +static void
> +insn_vops (basic_block bb, tree *vop_at_entry, tree *vop_at_exit)
> 
> it's easier to start from the bb end and walk until you see the
> first vdef or vuse.  Then you have *vop_at_exit.  From there
> just walk the SSA_NAME_DEF_STMTs of the vuse until you
> hit one whose definition is not in BB - and you have *vop_at_entry.
> That way you can avoid scanning most of the stmts.
> 
> The function also has an odd name ;)  It should be something like
> vops_at_bb_entry_and_exit.
> 
> +static void
> +vop_at_entry (basic_block bb1, basic_block bb2, tree *vop_at_entry1,
> +             tree *vop_at_entry2)
> 
> so you don't need the vop at exit at all?  The function is a bit unclear
> to me given it does so much stuff other than just computing the BBs
> entry VOPs ...
> 

Right, I overdid that a bit. I now have a simpler function vop_at_entry, which
only tries to calculate vop_at_entry.

> +static void
> +replace_block_by (basic_block bb1, basic_block bb2, bool update_vops)
> +{
> 
> can you add some comments before the different phases of update?
> I _think_ I understand what it does, but ...
> 

Ok, added some comments there, I hope that clarifies things.

> +/* Runs tail merge optimization.  */
> +
> +unsigned int
> +tail_merge_optimize (unsigned int todo)
> +{
> +  int nr_bbs_removed_total = 0;
> +  int nr_bbs_removed;
> +  bool loop_entered = false;
> +  int iteration_nr = 0;
> +  bool update_vops = ((todo & TODO_update_ssa_only_virtuals) == 0
> +                     || !symbol_marked_for_renaming (gimple_vop (cfun)));
> 
> you need to simplify this to
> 
>   bool update_vops = !symbol_marked_for_renaming (gimple_vop (cfun));
> 

Done.

> +      if (nr_bbs_removed == 0)
> +       break;
> +
> +      free_dominance_info (CDI_DOMINATORS);
> 
> we might want to limit the number of iterations we perform - especially
> as you are re-computing dominators on each iteration.  What's the
> maximum number of iterations you see during a GCC bootstrap?
> 

The maximum number is 16, which occurred 4 times, 2 times for
insn_default_latency_bdver1 and 2 times for internal_dfa_insn_code_bdver1,

histogram of iteration_nr:
0  139971
1   70248
2    2416
3     142
4      32
5       2
6       0
...
15      0
16      4

> +  if (dump_file)
> +    fprintf (dump_file, "htab collision / search: %f\n",
> +            htab_collisions (same_succ_htab));
> 
> in general without dump_flags & TDF_DETAILS passes should print
> at most things when they did a transformation (some even don't do that).
> Please double-check all your dump-prints.
> 

Done.

> +      todo |= (TODO_verify_ssa | TODO_verify_stmts | TODO_verify_flow
> +              | TODO_dump_func);
> 
> should all be already set.

Only TODO_verify_flow is always set by pre (and I think that's unnecessary if
nothing changed).

> 
> @ -4945,6 +4944,9 @@ execute_pre (bool do_fre)
>    scev_finalize ();
>    fini_pre (do_fre);
> 
> +  todo |= tail_merge_optimize (todo);
> +  free_scc_vn ();
> 
> Please only run tail_merge_optimize once.  As we are running through
> this code three times at -O2.  I suggest to try it in the !do_fre case
> only which we only run once (as PRE).  

Done.

> If that doesn't work out
> nicely we need to find other means (like introduce a
> pass_fre_and_tail_merge which passes down another flag and replace
> one FRE pass with that new combined pass).
> 
> Index: gcc/tree-flow.h
> ===================================================================
> --- gcc/tree-flow.h     (revision 175801)
> +++ gcc/tree-flow.h     (working copy)
> @@ -806,6 +806,9 @@ bool multiplier_allowed_in_address_p (HO
>  unsigned multiply_by_cost (HOST_WIDE_INT, enum machine_mode, bool);
>  bool may_be_nonaddressable_p (tree expr);
> 
> +/* In tree-ssa-tail-merge.c.  */
> +extern unsigned int tail_merge_optimize (unsigned int);
> 
> Eh, tree-flow.h kitchen-sink ;)  Please put it into tree-pass.h instead.
> 

Done.

> That said - I'm reasonably happy with the pass now,

Great :)

> but it's rather large
> (this review took 40min again ...) so I appreciate a second look from
> somebody else.

Thanks a lot for the review. I'll ask Ian Taylor in a separate email for review.

> 
> Btw, can you expand a bit on the amount of testcases?
> 

Increased amount of test-cases from 2 to 4. Will send them in the test-case thread.

> Thanks,
> Richard.

Runtime and impact on cc1:

               real        user        sys
without  21m21.308s  20m25.640s  0m27.540s
with     21m12.092s  20m22.540s  0m28.590s
                     ----------
                          -3.1s
                         -0.25%

$ size without/cc1 with/cc1
    text   data      bss       dec      hex     filename
17836921  41856  1351264  19230041  1256d59  without/cc1
17531665  41856  1351264  18924785  120c4f1     with/cc1
--------
 -305256
   -1.71%

I did some additional changes:
1. I added disregarding the order of local calculations (see local_def and
   uses).
2. I separated out the checking of availability of dependences
   (see deps_ok_for_redirect). This was previously checked while matching.
3. I keep a rep_bb for clusters. This is a bug fix, previous we just
   used the first bb of the clusters, and sometimes this resulted in
   uses not being dominated by their defs. This bug surfaced once I added 1.

Bootstrapped & reg-tested on x86_64.

Thanks again,
- Tom

2011-08-19  Tom de Vries  <tom@codesourcery.com>

	PR middle-end/43864
	* tree-ssa-tail-merge.c: New file.
	(struct same_succ): Define.
	(same_succ_t, const_same_succ_t): New typedef.
	(struct bb_cluster): Define.
	(bb_cluster_t, const_bb_cluster_t): New typedef.
	(struct aux_bb_info): Define.
	(BB_SIZE, BB_SAME_SUCC, BB_CLUSTER, BB_VOP_AT_EXIT): Define.
	(gvn_uses_equal): New function.
	(same_succ_print, same_succ_print_traverse, update_dep_bb)
	(stmt_update_dep_bb, local_def, same_succ_hash)
	(inverse_flags, same_succ_equal, same_succ_alloc, same_succ_delete)
	(same_succ_reset): New function.
	(same_succ_htab, same_succ_edge_flags)
	(deleted_bbs, deleted_bb_preds): New var.
	(debug_same_succ): New function.
	(worklist): New var.
	(print_worklist, add_to_worklist, find_same_succ_bb, find_same_succ)
	(init_worklist, delete_worklist, delete_basic_block_same_succ)
	(same_succ_flush_bbs, purge_bbs, update_worklist): New function.
	(print_cluster, debug_cluster, update_rep_bb)
	(add_bb_to_cluster, new_cluster, delete_cluster): New function.
	(all_clusters): New var.
	(alloc_cluster_vectors, reset_cluster_vectors, delete_cluster_vectors)
	(merge_clusters, set_cluster): New function.
	(gimple_equal_p, gsi_advance_bw_nondebug_nonlocal, find_duplicate)
	(same_phi_alternatives_1, same_phi_alternatives, bb_has_non_vop_phi)
	(deps_ok_for_redirect_from_bb_to_bb, deps_ok_for_redirect)
	(find_clusters_1, find_clusters): New function.
	(update_vuses, vop_phi, vop_at_entry, replace_block_by): New function.
	(update_bbs): New var.
	(apply_clusters): New function.
	(update_debug_stmt, update_debug_stmts): New function.
	(tail_merge_optimize): New function.
	tree-pass.h (tail_merge_optimize): Declare.
	* tree-ssa-pre.c (execute_pre): Use tail_merge_optimize.
	* Makefile.in (OBJS-common): Add tree-ssa-tail-merge.o.
	(tree-ssa-tail-merge.o): New rule.
	* opts.c (default_options_table): Set OPT_ftree_tail_merge by default at
	OPT_LEVELS_2_PLUS.
	* tree-ssa-sccvn.c (vn_valueize): Move to ...
	* tree-ssa-sccvn.h (vn_valueize): Here.
	* timevar.def (TV_TREE_TAIL_MERGE): New timevar.
	* common.opt (ftree-tail-merge): New switch.
	* params.def (PARAM_MAX_TAIL_MERGE_COMPARISONS): New parameter.
	* doc/invoke.texi (Optimization Options, -O2): Add -ftree-tail-merge.
	(-ftree-tail-merge, max-tail-merge-comparisons): New item.

Patch

Index: gcc/tree-ssa-tail-merge.c
===================================================================
--- gcc/tree-ssa-tail-merge.c	(revision 0)
+++ gcc/tree-ssa-tail-merge.c	(revision 0)
@@ -0,0 +1,1701 @@ 
+/* Tail merging for gimple.
+   Copyright (C) 2011 Free Software Foundation, Inc.
+   Contributed by Tom de Vries (tom@codesourcery.com)
+
+This file is part of GCC.
+
+GCC is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 3, or (at your option)
+any later version.
+
+GCC is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with GCC; see the file COPYING3.  If not see
+<http://www.gnu.org/licenses/>.  */
+
+/* Pass overview.
+
+
+   MOTIVATIONAL EXAMPLE
+
+   gimple representation of gcc/testsuite/gcc.dg/pr43864.c at
+
+   hprofStartupp (charD.1 * outputFileNameD.2600, charD.1 * ctxD.2601)
+   {
+     struct FILED.1638 * fpD.2605;
+     charD.1 fileNameD.2604[1000];
+     intD.0 D.3915;
+     const charD.1 * restrict outputFileName.0D.3914;
+
+     # BLOCK 2 freq:10000
+     # PRED: ENTRY [100.0%]  (fallthru,exec)
+     # PT = nonlocal { D.3926 } (restr)
+     outputFileName.0D.3914_3
+       = (const charD.1 * restrict) outputFileNameD.2600_2(D);
+     # .MEMD.3923_13 = VDEF <.MEMD.3923_12(D)>
+     # USE = nonlocal null { fileNameD.2604 D.3926 } (restr)
+     # CLB = nonlocal null { fileNameD.2604 D.3926 } (restr)
+     sprintfD.759 (&fileNameD.2604, outputFileName.0D.3914_3);
+     # .MEMD.3923_14 = VDEF <.MEMD.3923_13>
+     # USE = nonlocal null { fileNameD.2604 D.3926 } (restr)
+     # CLB = nonlocal null { fileNameD.2604 D.3926 } (restr)
+     D.3915_4 = accessD.2606 (&fileNameD.2604, 1);
+     if (D.3915_4 == 0)
+       goto <bb 3>;
+     else
+       goto <bb 4>;
+     # SUCC: 3 [10.0%]  (true,exec) 4 [90.0%]  (false,exec)
+
+     # BLOCK 3 freq:1000
+     # PRED: 2 [10.0%]  (true,exec)
+     # .MEMD.3923_15 = VDEF <.MEMD.3923_14>
+     # USE = nonlocal null { fileNameD.2604 D.3926 } (restr)
+     # CLB = nonlocal null { fileNameD.2604 D.3926 } (restr)
+     freeD.898 (ctxD.2601_5(D));
+     goto <bb 7>;
+     # SUCC: 7 [100.0%]  (fallthru,exec)
+
+     # BLOCK 4 freq:9000
+     # PRED: 2 [90.0%]  (false,exec)
+     # .MEMD.3923_16 = VDEF <.MEMD.3923_14>
+     # PT = nonlocal escaped
+     # USE = nonlocal null { fileNameD.2604 D.3926 } (restr)
+     # CLB = nonlocal null { fileNameD.2604 D.3926 } (restr)
+     fpD.2605_8 = fopenD.1805 (&fileNameD.2604[0], 0B);
+     if (fpD.2605_8 == 0B)
+       goto <bb 5>;
+     else
+       goto <bb 6>;
+     # SUCC: 5 [1.9%]  (true,exec) 6 [98.1%]  (false,exec)
+
+     # BLOCK 5 freq:173
+     # PRED: 4 [1.9%]  (true,exec)
+     # .MEMD.3923_17 = VDEF <.MEMD.3923_16>
+     # USE = nonlocal null { fileNameD.2604 D.3926 } (restr)
+     # CLB = nonlocal null { fileNameD.2604 D.3926 } (restr)
+     freeD.898 (ctxD.2601_5(D));
+     goto <bb 7>;
+     # SUCC: 7 [100.0%]  (fallthru,exec)
+
+     # BLOCK 6 freq:8827
+     # PRED: 4 [98.1%]  (false,exec)
+     # .MEMD.3923_18 = VDEF <.MEMD.3923_16>
+     # USE = nonlocal null { fileNameD.2604 D.3926 } (restr)
+     # CLB = nonlocal null { fileNameD.2604 D.3926 } (restr)
+     fooD.2599 (outputFileNameD.2600_2(D), fpD.2605_8);
+     # SUCC: 7 [100.0%]  (fallthru,exec)
+
+     # BLOCK 7 freq:10000
+     # PRED: 3 [100.0%]  (fallthru,exec) 5 [100.0%]  (fallthru,exec)
+             6 [100.0%]  (fallthru,exec)
+     # PT = nonlocal null
+
+     # ctxD.2601_1 = PHI <0B(3), 0B(5), ctxD.2601_5(D)(6)>
+     # .MEMD.3923_11 = PHI <.MEMD.3923_15(3), .MEMD.3923_17(5),
+                            .MEMD.3923_18(6)>
+     # VUSE <.MEMD.3923_11>
+     return ctxD.2601_1;
+     # SUCC: EXIT [100.0%]
+   }
+
+   bb 3 and bb 5 can be merged.  The blocks have different predecessors, but the
+   same successors, and the same operations.
+
+
+   CONTEXT
+
+   A technique called tail merging (or cross jumping) can fix the example
+   above.  For a block, we look for common code at the end (the tail) of the
+   predecessor blocks, and insert jumps from one block to the other.
+   The example is a special case for tail merging, in that 2 whole blocks
+   can be merged, rather than just the end parts of it.
+   We currently only focus on whole block merging, so in that sense
+   calling this pass tail merge is a bit of a misnomer.
+
+   We distinguish 2 kinds of situations in which blocks can be merged:
+   - same operations, same predecessors.  The successor edges coming from one
+     block are redirected to come from the other block.
+   - same operations, same successors.  The predecessor edges entering one block
+     are redirected to enter the other block.  Note that this operation might
+     involve introducing phi operations.
+
+   For efficient implementation, we would like to value numbers the blocks, and
+   have a comparison operator that tells us whether the blocks are equal.
+   Besides being runtime efficient, block value numbering should also abstract
+   from irrelevant differences in order of operations, much like normal value
+   numbering abstracts from irrelevant order of operations.
+
+   For the first situation (same_operations, same predecessors), normal value
+   numbering fits well.  We can calculate a block value number based on the
+   value numbers of the defs and vdefs.
+
+   For the second situation (same operations, same successors), this approach
+   doesn't work so well.  We can illustrate this using the example.  The calls
+   to free use different vdefs: MEMD.3923_16 and MEMD.3923_14, and these will
+   remain different in value numbering, since they represent different memory
+   states.  So the resulting vdefs of the frees will be different in value
+   numbering, so the block value numbers will be different.
+
+   The reason why we call the blocks equal is not because they define the same
+   values, but because uses in the blocks use (possibly different) defs in the
+   same way.  To be able to detect this efficiently, we need to do some kind of
+   reverse value numbering, meaning number the uses rather than the defs, and
+   calculate a block value number based on the value number of the uses.
+   Ideally, a block comparison operator will also indicate which phis are needed
+   to merge the blocks.
+
+   For the moment, we don't do block value numbering, but we do insn-by-insn
+   matching, using scc value numbers to match operations with results, and
+   structural comparison otherwise, while ignoring vop mismatches.
+
+
+   IMPLEMENTATION
+
+   1. The pass first determines all groups of blocks with the same successor
+      blocks.
+   2. Within each group, it tries to determine clusters of equal basic blocks.
+   3. The clusters are applied.
+   4. The same successor groups are updated.
+   5. This process is repeated from 2 onwards, until no more changes.
+
+
+   LIMITATIONS/TODO
+
+   - block only
+   - handles only 'same operations, same successors'.
+     It handles same predecessors as a special subcase though.
+   - does not implement the reverse value numbering and block value numbering.
+   - improve memory allocation: use garbage collected memory, obstacks,
+     allocpools where appropriate.
+   - no insertion of gimple_reg phis,  We only introduce vop-phis.
+   - handle blocks with gimple_reg phi_nodes.
+
+
+   SWITCHES
+
+   - ftree-tail-merge.  On at -O2.  We may have to enable it only at -Os.  */
+
+#include "config.h"
+#include "system.h"
+#include "coretypes.h"
+#include "tm.h"
+#include "tree.h"
+#include "tm_p.h"
+#include "basic-block.h"
+#include "output.h"
+#include "flags.h"
+#include "function.h"
+#include "tree-flow.h"
+#include "timevar.h"
+#include "bitmap.h"
+#include "tree-ssa-alias.h"
+#include "params.h"
+#include "tree-pretty-print.h"
+#include "hashtab.h"
+#include "gimple-pretty-print.h"
+#include "tree-ssa-sccvn.h"
+#include "tree-dump.h"
+
+/* Describes a group of bbs with the same successors.  The successor bbs are
+   cached in succs, and the successor edge flags are cached in succ_flags.
+   If a bb has the EDGE_TRUE/VALSE_VALUE flags swapped compared to succ_flags,
+   it's marked in inverse.
+   Additionally, the hash value for the struct is cached in hashval, and
+   in_worklist indicates whether it's currently part of worklist.  */
+
+struct same_succ
+{
+  /* The bbs that have the same successor bbs.  */
+  bitmap bbs;
+  /* The successor bbs.  */
+  bitmap succs;
+  /* Indicates whether the EDGE_TRUE/FALSE_VALUEs of succ_flags are swapped for
+     bb.  */
+  bitmap inverse;
+  /* The edge flags for each of the successor bbs.  */
+  VEC (int, heap) *succ_flags;
+  /* Indicates whether the struct is currently in the worklist.  */
+  bool in_worklist;
+  /* The hash value of the struct.  */
+  hashval_t hashval;
+};
+typedef struct same_succ *same_succ_t;
+typedef const struct same_succ *const_same_succ_t;
+
+/* A group of bbs where 1 bb from bbs can replace the other bbs.  */
+
+struct bb_cluster
+{
+  /* The bbs in the cluster.  */
+  bitmap bbs;
+  /* The preds of the bbs in the cluster.  */
+  bitmap preds;
+  /* Index in all_clusters vector.  */
+  int index;
+  /* The bb to replace the cluster with.  */
+  basic_block rep_bb;
+};
+typedef struct bb_cluster *bb_cluster_t;
+typedef const struct bb_cluster *const_bb_cluster_t;
+
+/* Per bb-info.  */
+
+struct aux_bb_info
+{
+  /* The number of non-debug statements in the bb.  */
+  int size;
+  /* The same_succ that this bb is a member of.  */
+  same_succ_t same_succ;
+  /* The cluster that this bb is a member of.  */
+  bb_cluster_t cluster;
+  /* The vop state at the exit of a bb.  This is shortlived data, used to
+     communicate data between update_block_by and update_vuses.  */
+  tree vop_at_exit;
+  /* The bb that either contains or is dominated by the dependencies of the
+     bb.  */
+  basic_block dep_bb;
+};
+
+/* Macros to access the fields of struct aux_bb_info.  */
+
+#define BB_SIZE(bb) (((struct aux_bb_info *)bb->aux)->size)
+#define BB_SAME_SUCC(bb) (((struct aux_bb_info *)bb->aux)->same_succ)
+#define BB_CLUSTER(bb) (((struct aux_bb_info *)bb->aux)->cluster)
+#define BB_VOP_AT_EXIT(bb) (((struct aux_bb_info *)bb->aux)->vop_at_exit)
+#define BB_DEP_BB(bb) (((struct aux_bb_info *)bb->aux)->dep_bb)
+
+/* VAL1 and VAL2 are either:
+   - uses in BB1 and BB2, or
+   - phi alternatives for BB1 and BB2.
+   Return true if the uses have the same gvn value.  */
+
+static bool
+gvn_uses_equal (tree val1, tree val2)
+{
+  gcc_checking_assert (val1 != NULL_TREE && val2 != NULL_TREE);
+
+  if (val1 == val2)
+    return true;
+
+  if (vn_valueize (val1) != vn_valueize (val2))
+    return false;
+
+  return ((TREE_CODE (val1) == SSA_NAME || CONSTANT_CLASS_P (val1))
+	  && (TREE_CODE (val2) == SSA_NAME || CONSTANT_CLASS_P (val2)));
+}
+
+/* Prints E to FILE.  */
+
+static void
+same_succ_print (FILE *file, const same_succ_t e)
+{
+  unsigned int i;
+  bitmap_print (file, e->bbs, "bbs:", "\n");
+  bitmap_print (file, e->succs, "succs:", "\n");
+  bitmap_print (file, e->inverse, "inverse:", "\n");
+  fprintf (file, "flags:");
+  for (i = 0; i < VEC_length (int, e->succ_flags); ++i)
+    fprintf (file, " %x", VEC_index (int, e->succ_flags, i));
+  fprintf (file, "\n");
+}
+
+/* Prints same_succ VE to VFILE.  */
+
+static int
+same_succ_print_traverse (void **ve, void *vfile)
+{
+  const same_succ_t e = *((const same_succ_t *)ve);
+  FILE *file = ((FILE*)vfile);
+  same_succ_print (file, e);
+  return 1;
+}
+
+/* Update BB_DEP_BB (USE_BB), given a use of VAL in USE_BB.  */
+
+static void
+update_dep_bb (basic_block use_bb, tree val)
+{
+  basic_block dep_bb;
+
+  /* Not a dep.  */
+  if (TREE_CODE (val) != SSA_NAME)
+    return;
+
+  /* Skip use of global def.  */
+  if (SSA_NAME_IS_DEFAULT_DEF (val))
+    return;
+
+  /* Skip use of local def.  */
+  dep_bb = gimple_bb (SSA_NAME_DEF_STMT (val));
+  if (dep_bb == use_bb)
+    return;
+
+  if (BB_DEP_BB (use_bb) == NULL
+      || dominated_by_p (CDI_DOMINATORS, dep_bb, BB_DEP_BB (use_bb)))
+    BB_DEP_BB (use_bb) = dep_bb;
+}
+
+/* Update BB_DEP_BB, given the dependencies in STMT.  */
+
+static void
+stmt_update_dep_bb (gimple stmt)
+{
+  ssa_op_iter iter;
+  use_operand_p use;
+
+  FOR_EACH_SSA_USE_OPERAND (use, stmt, iter, SSA_OP_USE)
+    update_dep_bb (gimple_bb (stmt), USE_FROM_PTR (use));
+}
+
+/* Returns whether VAL is used in the same bb as in which it is defined, or
+   in the phi of a successor bb.  */
+
+static bool
+local_def (tree val)
+{
+  gimple stmt, def_stmt;
+  basic_block bb, def_bb;
+  imm_use_iterator iter;
+  bool res;
+
+  if (TREE_CODE (val) != SSA_NAME)
+    return false;
+  def_stmt = SSA_NAME_DEF_STMT (val);
+  def_bb = gimple_bb (def_stmt);
+
+  res = true;
+  FOR_EACH_IMM_USE_STMT (stmt, iter, val)
+    {
+      bb = gimple_bb (stmt);
+      if (bb == def_bb)
+	continue;
+      if (gimple_code (stmt) == GIMPLE_PHI
+	  && find_edge (def_bb, bb))
+	continue;
+      res = false;
+      BREAK_FROM_IMM_USE_STMT (iter);
+    }
+  return res;
+}
+
+/* Calculates hash value for same_succ VE.  */
+
+static hashval_t
+same_succ_hash (const void *ve)
+{
+  const_same_succ_t e = (const_same_succ_t)ve;
+  hashval_t hashval = bitmap_hash (e->succs);
+  int flags;
+  unsigned int i;
+  unsigned int first = bitmap_first_set_bit (e->bbs);
+  basic_block bb = BASIC_BLOCK (first);
+  int size = 0;
+  gimple_stmt_iterator gsi;
+  gimple stmt;
+  tree arg;
+  unsigned int s;
+  bitmap_iterator bs;
+
+  for (gsi = gsi_start_nondebug_bb (bb);
+       !gsi_end_p (gsi); gsi_next_nondebug (&gsi))
+    {
+      stmt = gsi_stmt (gsi);
+      stmt_update_dep_bb (stmt);
+      if (is_gimple_assign (stmt) && local_def (gimple_get_lhs (stmt))
+	  && !gimple_has_side_effects (stmt))
+	continue;
+      size++;
+
+      hashval = iterative_hash_hashval_t (gimple_code (stmt), hashval);
+      if (is_gimple_assign (stmt))
+	hashval = iterative_hash_hashval_t (gimple_assign_rhs_code (stmt),
+					    hashval);
+      if (!is_gimple_call (stmt))
+	continue;
+      if (gimple_call_internal_p (stmt))
+	hashval = iterative_hash_hashval_t
+	  ((hashval_t) gimple_call_internal_fn (stmt), hashval);
+      else
+	hashval = iterative_hash_expr (gimple_call_fn (stmt), hashval);
+      for (i = 0; i < gimple_call_num_args (stmt); i++)
+	{
+	  arg = gimple_call_arg (stmt, i);
+	  arg = vn_valueize (arg);
+	  hashval = iterative_hash_expr (arg, hashval);
+	}
+    }
+
+  hashval = iterative_hash_hashval_t (size, hashval);
+  BB_SIZE (bb) = size;
+
+  for (i = 0; i < VEC_length (int, e->succ_flags); ++i)
+    {
+      flags = VEC_index (int, e->succ_flags, i);
+      flags = flags & ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
+      hashval = iterative_hash_hashval_t (flags, hashval);
+    }
+
+  EXECUTE_IF_SET_IN_BITMAP (e->succs, 0, s, bs)
+    {
+      int n = find_edge (bb, BASIC_BLOCK (s))->dest_idx;
+      for (gsi = gsi_start_phis (BASIC_BLOCK (s)); !gsi_end_p (gsi);
+	   gsi_next (&gsi))
+	{
+	  gimple phi = gsi_stmt (gsi);
+	  tree lhs = gimple_phi_result (phi);
+	  tree val = gimple_phi_arg_def (phi, n);
+
+	  if (!is_gimple_reg (lhs))
+	    continue;
+	  update_dep_bb (bb, val);
+	}
+    }
+
+  return hashval;
+}
+
+/* Returns true if E1 and E2 have 2 successors, and if the successor flags
+   are inverse for the EDGE_TRUE_VALUE and EDGE_FALSE_VALUE flags, and equal for
+   the other edge flags.  */
+
+static bool
+inverse_flags (const_same_succ_t e1, const_same_succ_t e2)
+{
+  int f1a, f1b, f2a, f2b;
+  int mask = ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
+
+  if (VEC_length (int, e1->succ_flags) != 2)
+    return false;
+
+  f1a = VEC_index (int, e1->succ_flags, 0);
+  f1b = VEC_index (int, e1->succ_flags, 1);
+  f2a = VEC_index (int, e2->succ_flags, 0);
+  f2b = VEC_index (int, e2->succ_flags, 1);
+
+  if (f1a == f2a && f1b == f2b)
+    return false;
+
+  return (f1a & mask) == (f2a & mask) && (f1b & mask) == (f2b & mask);
+}
+
+/* Compares SAME_SUCCs VE1 and VE2.  */
+
+static int
+same_succ_equal (const void *ve1, const void *ve2)
+{
+  const_same_succ_t e1 = (const_same_succ_t)ve1;
+  const_same_succ_t e2 = (const_same_succ_t)ve2;
+  unsigned int i, first1, first2;
+  gimple_stmt_iterator gsi1, gsi2;
+  gimple s1, s2;
+  basic_block bb1, bb2;
+
+  if (e1->hashval != e2->hashval)
+    return 0;
+
+  if (VEC_length (int, e1->succ_flags) != VEC_length (int, e2->succ_flags))
+    return 0;
+
+  if (!bitmap_equal_p (e1->succs, e2->succs))
+    return 0;
+
+  if (!inverse_flags (e1, e2))
+    {
+      for (i = 0; i < VEC_length (int, e1->succ_flags); ++i)
+	if (VEC_index (int, e1->succ_flags, i)
+	    != VEC_index (int, e1->succ_flags, i))
+	  return 0;
+    }
+
+  first1 = bitmap_first_set_bit (e1->bbs);
+  first2 = bitmap_first_set_bit (e2->bbs);
+
+  bb1 = BASIC_BLOCK (first1);
+  bb2 = BASIC_BLOCK (first2);
+
+  if (BB_SIZE (bb1) != BB_SIZE (bb2))
+    return 0;
+
+  gsi1 = gsi_start_nondebug_bb (bb1);
+  gsi2 = gsi_start_nondebug_bb (bb2);
+  while (!(gsi_end_p (gsi1) || gsi_end_p (gsi2)))
+    {
+      s1 = gsi_stmt (gsi1);
+      s2 = gsi_stmt (gsi2);
+      if (gimple_code (s1) != gimple_code (s2))
+	return 0;
+      if (is_gimple_call (s1) && !gimple_call_same_target_p (s1, s2))
+	return 0;
+      gsi_next_nondebug (&gsi1);
+      gsi_next_nondebug (&gsi2);
+    }
+
+  return 1;
+}
+
+/* Alloc and init a new SAME_SUCC.  */
+
+static same_succ_t
+same_succ_alloc (void)
+{
+  same_succ_t same = XNEW (struct same_succ);
+
+  same->bbs = BITMAP_ALLOC (NULL);
+  same->succs = BITMAP_ALLOC (NULL);
+  same->inverse = BITMAP_ALLOC (NULL);
+  same->succ_flags = VEC_alloc (int, heap, 10);
+  same->in_worklist = false;
+
+  return same;
+}
+
+/* Delete same_succ VE.  */
+
+static void
+same_succ_delete (void *ve)
+{
+  same_succ_t e = (same_succ_t)ve;
+
+  BITMAP_FREE (e->bbs);
+  BITMAP_FREE (e->succs);
+  BITMAP_FREE (e->inverse);
+  VEC_free (int, heap, e->succ_flags);
+
+  XDELETE (ve);
+}
+
+/* Reset same_succ SAME.  */
+
+static void
+same_succ_reset (same_succ_t same)
+{
+  bitmap_clear (same->bbs);
+  bitmap_clear (same->succs);
+  bitmap_clear (same->inverse);
+  VEC_truncate (int, same->succ_flags, 0);
+}
+
+/* Hash table with all same_succ entries.  */
+
+static htab_t same_succ_htab;
+
+/* Array that is used to store the edge flags for a successor.  */
+
+static int *same_succ_edge_flags;
+
+/* Bitmap that is used to mark bbs that are recently deleted.  */
+
+static bitmap deleted_bbs;
+
+/* Bitmap that is used to mark predecessors of bbs that are
+   deleted.  */
+
+static bitmap deleted_bb_preds;
+
+/* Prints same_succ_htab to stderr.  */
+
+extern void debug_same_succ (void);
+DEBUG_FUNCTION void
+debug_same_succ ( void)
+{
+  htab_traverse (same_succ_htab, same_succ_print_traverse, stderr);
+}
+
+DEF_VEC_P (same_succ_t);
+DEF_VEC_ALLOC_P (same_succ_t, heap);
+
+/* Vector of bbs to process.  */
+
+static VEC (same_succ_t, heap) *worklist;
+
+/* Prints worklist to FILE.  */
+
+static void
+print_worklist (FILE *file)
+{
+  unsigned int i;
+  for (i = 0; i < VEC_length (same_succ_t, worklist); ++i)
+    same_succ_print (file, VEC_index (same_succ_t, worklist, i));
+}
+
+/* Adds SAME to worklist.  */
+
+static void
+add_to_worklist (same_succ_t same)
+{
+  if (same->in_worklist)
+    return;
+
+  if (bitmap_count_bits (same->bbs) < 2)
+    return;
+
+  same->in_worklist = true;
+  VEC_safe_push (same_succ_t, heap, worklist, same);
+}
+
+/* Add BB to same_succ_htab.  */
+
+static void
+find_same_succ_bb (basic_block bb, same_succ_t *same_p)
+{
+  unsigned int j;
+  bitmap_iterator bj;
+  same_succ_t same = *same_p;
+  same_succ_t *slot;
+  edge_iterator ei;
+  edge e;
+
+  if (bb == NULL)
+    return;
+  bitmap_set_bit (same->bbs, bb->index);
+  FOR_EACH_EDGE (e, ei, bb->succs)
+    {
+      int index = e->dest->index;
+      bitmap_set_bit (same->succs, index);
+      same_succ_edge_flags[index] = e->flags;
+    }
+  EXECUTE_IF_SET_IN_BITMAP (same->succs, 0, j, bj)
+    VEC_safe_push (int, heap, same->succ_flags, same_succ_edge_flags[j]);
+
+  same->hashval = same_succ_hash (same);
+
+  slot = (same_succ_t *) htab_find_slot_with_hash (same_succ_htab, same,
+						   same->hashval, INSERT);
+  if (*slot == NULL)
+    {
+      *slot = same;
+      BB_SAME_SUCC (bb) = same;
+      add_to_worklist (same);
+      *same_p = NULL;
+    }
+  else
+    {
+      bitmap_set_bit ((*slot)->bbs, bb->index);
+      BB_SAME_SUCC (bb) = *slot;
+      add_to_worklist (*slot);
+      if (inverse_flags (same, *slot))
+	bitmap_set_bit ((*slot)->inverse, bb->index);
+      same_succ_reset (same);
+    }
+}
+
+/* Find bbs with same successors.  */
+
+static void
+find_same_succ (void)
+{
+  same_succ_t same = same_succ_alloc ();
+  basic_block bb;
+
+  FOR_EACH_BB (bb)
+    {
+      find_same_succ_bb (bb, &same);
+      if (same == NULL)
+	same = same_succ_alloc ();
+    }
+
+  same_succ_delete (same);
+}
+
+/* Initializes worklist administration.  */
+
+static void
+init_worklist (void)
+{
+  alloc_aux_for_blocks (sizeof (struct aux_bb_info));
+  same_succ_htab
+    = htab_create (n_basic_blocks, same_succ_hash, same_succ_equal,
+		   same_succ_delete);
+  same_succ_edge_flags = XCNEWVEC (int, last_basic_block);
+  deleted_bbs = BITMAP_ALLOC (NULL);
+  deleted_bb_preds = BITMAP_ALLOC (NULL);
+  worklist = VEC_alloc (same_succ_t, heap, n_basic_blocks);
+  find_same_succ ();
+
+  if (dump_file && (dump_flags & TDF_DETAILS))
+    {
+      fprintf (dump_file, "initial worklist:\n");
+      print_worklist (dump_file);
+    }
+}
+
+/* Deletes worklist administration.  */
+
+static void
+delete_worklist (void)
+{
+  free_aux_for_blocks ();
+  htab_delete (same_succ_htab);
+  same_succ_htab = NULL;
+  XDELETEVEC (same_succ_edge_flags);
+  same_succ_edge_flags = NULL;
+  BITMAP_FREE (deleted_bbs);
+  BITMAP_FREE (deleted_bb_preds);
+  VEC_free (same_succ_t, heap, worklist);
+}
+
+/* Mark BB as deleted, and mark its predecessors.  */
+
+static void
+delete_basic_block_same_succ (basic_block bb)
+{
+  edge e;
+  edge_iterator ei;
+
+  bitmap_set_bit (deleted_bbs, bb->index);
+
+  FOR_EACH_EDGE (e, ei, bb->preds)
+    bitmap_set_bit (deleted_bb_preds, e->src->index);
+}
+
+/* Removes all bbs in BBS from their corresponding same_succ.  */
+
+static void
+same_succ_flush_bbs (bitmap bbs)
+{
+  unsigned int i;
+  bitmap_iterator bi;
+
+  EXECUTE_IF_SET_IN_BITMAP (bbs, 0, i, bi)
+    {
+      basic_block bb = BASIC_BLOCK (i);
+      same_succ_t same = BB_SAME_SUCC (bb);
+      BB_SAME_SUCC (bb) = NULL;
+      if (bitmap_single_bit_set_p (same->bbs))
+	htab_remove_elt_with_hash (same_succ_htab, same, same->hashval);
+      else
+	bitmap_clear_bit (same->bbs, i);
+    }
+}
+
+/* Delete all deleted_bbs.  */
+
+static void
+purge_bbs (void)
+{
+  unsigned int i;
+  bitmap_iterator bi;
+
+  same_succ_flush_bbs (deleted_bbs);
+
+  EXECUTE_IF_SET_IN_BITMAP (deleted_bbs, 0, i, bi)
+    delete_basic_block (BASIC_BLOCK (i));
+
+  bitmap_and_compl_into (deleted_bb_preds, deleted_bbs);
+  bitmap_clear (deleted_bbs);
+}
+
+/* For deleted_bb_preds, find bbs with same successors.  */
+
+static void
+update_worklist (void)
+{
+  unsigned int i;
+  bitmap_iterator bi;
+  basic_block bb;
+  same_succ_t same;
+
+  bitmap_clear_bit (deleted_bb_preds, ENTRY_BLOCK);
+  same_succ_flush_bbs (deleted_bb_preds);
+
+  same = same_succ_alloc ();
+  EXECUTE_IF_SET_IN_BITMAP (deleted_bb_preds, 0, i, bi)
+    {
+      bb = BASIC_BLOCK (i);
+      gcc_assert (bb != NULL);
+      find_same_succ_bb (bb, &same);
+      if (same == NULL)
+	same = same_succ_alloc ();
+    }
+  same_succ_delete (same);
+  bitmap_clear (deleted_bb_preds);
+}
+
+/* Prints cluster C to FILE.  */
+
+static void
+print_cluster (FILE *file, bb_cluster_t c)
+{
+  if (c == NULL)
+    return;
+  bitmap_print (file, c->bbs, "bbs:", "\n");
+  bitmap_print (file, c->preds, "preds:", "\n");
+}
+
+/* Prints cluster C to stderr.  */
+
+extern void debug_cluster (bb_cluster_t);
+DEBUG_FUNCTION void
+debug_cluster (bb_cluster_t c)
+{
+  print_cluster (stderr, c);
+}
+
+/* Update C->rep_bb, given that BB is added to the cluster.  */
+
+static void
+update_rep_bb (bb_cluster_t c, basic_block bb)
+{
+  /* Initial.  */
+  if (c->rep_bb == NULL)
+    {
+      c->rep_bb = bb;
+      return;
+    }
+
+  /* Current needs no deps, keep it.  */
+  if (BB_DEP_BB (c->rep_bb) == NULL)
+    return;
+
+  /* Bb needs no deps, change rep_bb.  */
+  if (BB_DEP_BB (bb) == NULL)
+    {
+      c->rep_bb = bb;
+      return;
+    }
+
+  /* Bb needs last deps earlier than current, change rep_bb.  A potential
+     problem with this, is that the first deps might also be earlier, which
+     would mean we prefer longer lifetimes for the deps.  To be able to check
+     for this, we would have to trace BB_FIRST_DEP_BB as well, besides
+     BB_DEP_BB, which is really BB_LAST_DEP_BB.
+     The benefit of choosing the bb with last deps earlier, is that it can
+     potentially be used as replacement for more bbs.  */
+  if (dominated_by_p (CDI_DOMINATORS, BB_DEP_BB (c->rep_bb), BB_DEP_BB (bb)))
+    c->rep_bb = bb;
+}
+
+/* Add BB to cluster C.  Sets BB in C->bbs, and preds of BB in C->preds.  */
+
+static void
+add_bb_to_cluster (bb_cluster_t c, basic_block bb)
+{
+  edge e;
+  edge_iterator ei;
+
+  bitmap_set_bit (c->bbs, bb->index);
+
+  FOR_EACH_EDGE (e, ei, bb->preds)
+    bitmap_set_bit (c->preds, e->src->index);
+
+  update_rep_bb (c, bb);
+}
+
+/* Allocate and init new cluster.  */
+
+static bb_cluster_t
+new_cluster (void)
+{
+  bb_cluster_t c;
+  c = XCNEW (struct bb_cluster);
+  c->bbs = BITMAP_ALLOC (NULL);
+  c->preds = BITMAP_ALLOC (NULL);
+  c->rep_bb = NULL;
+  return c;
+}
+
+/* Delete clusters.  */
+
+static void
+delete_cluster (bb_cluster_t c)
+{
+  if (c == NULL)
+    return;
+  BITMAP_FREE (c->bbs);
+  BITMAP_FREE (c->preds);
+  XDELETE (c);
+}
+
+DEF_VEC_P (bb_cluster_t);
+DEF_VEC_ALLOC_P (bb_cluster_t, heap);
+
+/* Array that contains all clusters.  */
+
+static VEC (bb_cluster_t, heap) *all_clusters;
+
+/* Allocate all cluster vectors.  */
+
+static void
+alloc_cluster_vectors (void)
+{
+  all_clusters = VEC_alloc (bb_cluster_t, heap, n_basic_blocks);
+}
+
+/* Reset all cluster vectors.  */
+
+static void
+reset_cluster_vectors (void)
+{
+  unsigned int i;
+  basic_block bb;
+  for (i = 0; i < VEC_length (bb_cluster_t, all_clusters); ++i)
+    delete_cluster (VEC_index (bb_cluster_t, all_clusters, i));
+  VEC_truncate (bb_cluster_t, all_clusters, 0);
+  FOR_EACH_BB (bb)
+    BB_CLUSTER (bb) = NULL;
+}
+
+/* Delete all cluster vectors.  */
+
+static void
+delete_cluster_vectors (void)
+{
+  unsigned int i;
+  for (i = 0; i < VEC_length (bb_cluster_t, all_clusters); ++i)
+    delete_cluster (VEC_index (bb_cluster_t, all_clusters, i));
+  VEC_free (bb_cluster_t, heap, all_clusters);
+}
+
+/* Merge cluster C2 into C1.  */
+
+static void
+merge_clusters (bb_cluster_t c1, bb_cluster_t c2)
+{
+  bitmap_ior_into (c1->bbs, c2->bbs);
+  bitmap_ior_into (c1->preds, c2->preds);
+}
+
+/* Register equivalence of BB1 and BB2 (members of cluster C).  Store c in
+   all_clusters, or merge c with existing cluster.  */
+
+static void
+set_cluster (basic_block bb1, basic_block bb2)
+{
+  basic_block merge_bb, other_bb;
+  bb_cluster_t merge, old, c;
+
+  if (BB_CLUSTER (bb1) == NULL && BB_CLUSTER (bb2) == NULL)
+    {
+      c = new_cluster ();
+      add_bb_to_cluster (c, bb1);
+      add_bb_to_cluster (c, bb2);
+      BB_CLUSTER (bb1) = c;
+      BB_CLUSTER (bb2) = c;
+      c->index = VEC_length (bb_cluster_t, all_clusters);
+      VEC_safe_push (bb_cluster_t, heap, all_clusters, c);
+    }
+  else if (BB_CLUSTER (bb1) == NULL || BB_CLUSTER (bb2) == NULL)
+    {
+      merge_bb = BB_CLUSTER (bb1) == NULL ? bb2 : bb1;
+      other_bb = BB_CLUSTER (bb1) == NULL ? bb1 : bb2;
+      merge = BB_CLUSTER (merge_bb);
+      add_bb_to_cluster (merge, other_bb);
+      BB_CLUSTER (other_bb) = merge;
+    }
+  else if (BB_CLUSTER (bb1) != BB_CLUSTER (bb2))
+    {
+      unsigned int i;
+      bitmap_iterator bi;
+
+      old = BB_CLUSTER (bb2);
+      merge = BB_CLUSTER (bb1);
+      merge_clusters (merge, old);
+      EXECUTE_IF_SET_IN_BITMAP (old->bbs, 0, i, bi)
+	BB_CLUSTER (BASIC_BLOCK (i)) = merge;
+      VEC_replace (bb_cluster_t, all_clusters, old->index, NULL);
+      update_rep_bb (merge, old->rep_bb);
+      delete_cluster (old);
+    }
+  else
+    gcc_unreachable ();
+}
+
+/* Return true if gimple statements S1 and S2 are equal.  Gimple_bb (s1) and
+   gimple_bb (s2) are members of SAME_SUCC.  */
+
+static bool
+gimple_equal_p (same_succ_t same_succ, gimple s1, gimple s2)
+{
+  unsigned int i;
+  tree lhs1, lhs2;
+  basic_block bb1 = gimple_bb (s1), bb2 = gimple_bb (s2);
+  tree t1, t2;
+  bool equal, inv_cond;
+  enum tree_code code1, code2;
+
+  if (gimple_code (s1) != gimple_code (s2))
+    return false;
+
+  switch (gimple_code (s1))
+    {
+    case GIMPLE_CALL:
+      if (gimple_call_num_args (s1) != gimple_call_num_args (s2))
+	return false;
+      if (!gimple_call_same_target_p (s1, s2))
+        return false;
+
+      equal = true;
+      for (i = 0; i < gimple_call_num_args (s1); ++i)
+	{
+	  t1 = gimple_call_arg (s1, i);
+	  t2 = gimple_call_arg (s2, i);
+	  if (operand_equal_p (t1, t2, 0))
+	    continue;
+	  if (gvn_uses_equal (t1, t2))
+	    continue;
+	  equal = false;
+	  break;
+	}
+      if (equal)
+	return true;
+
+      lhs1 = gimple_get_lhs (s1);
+      lhs2 = gimple_get_lhs (s2);
+      return (lhs1 != NULL_TREE && lhs2 != NULL_TREE
+	      && TREE_CODE (lhs1) == SSA_NAME && TREE_CODE (lhs2) == SSA_NAME
+	      && vn_valueize (lhs1) == vn_valueize (lhs2));
+
+    case GIMPLE_ASSIGN:
+      lhs1 = gimple_get_lhs (s1);
+      lhs2 = gimple_get_lhs (s2);
+      return (TREE_CODE (lhs1) == SSA_NAME
+	      && TREE_CODE (lhs2) == SSA_NAME
+	      && vn_valueize (lhs1) == vn_valueize (lhs2));
+
+    case GIMPLE_COND:
+      t1 = gimple_cond_lhs (s1);
+      t2 = gimple_cond_lhs (s2);
+      if (!operand_equal_p (t1, t2, 0)
+	  && !gvn_uses_equal (t1, t2))
+	return false;
+
+      t1 = gimple_cond_rhs (s1);
+      t2 = gimple_cond_rhs (s2);
+      if (!operand_equal_p (t1, t2, 0)
+	  && !gvn_uses_equal (t1, t2))
+	return false;
+
+      code1 = gimple_expr_code (s1);
+      code2 = gimple_expr_code (s2);
+      inv_cond = (bitmap_bit_p (same_succ->inverse, bb1->index)
+		  != bitmap_bit_p (same_succ->inverse, bb2->index));
+      if (inv_cond)
+	{
+	  bool honor_nans
+	    = HONOR_NANS (TYPE_MODE (TREE_TYPE (gimple_cond_lhs (s1))));
+	  code2 = invert_tree_comparison (code2, honor_nans);
+	}
+      return code1 == code2;
+
+    default:
+      return false;
+    }
+}
+
+/* Let GSI skip backwards over local defs.  */
+
+static void
+gsi_advance_bw_nondebug_nonlocal (gimple_stmt_iterator *gsi)
+{
+  gimple stmt;
+
+  while (true)
+    {
+      if (gsi_end_p (*gsi))
+	return;
+      stmt = gsi_stmt (*gsi);
+      if (!(is_gimple_assign (stmt) && local_def (gimple_get_lhs (stmt))
+	    && !gimple_has_side_effects (stmt)))
+	return;
+      gsi_prev_nondebug (gsi);
+    }
+}
+
+/* Determines whether BB1 and BB2 (members of same_succ) are duplicates.  If so,
+   clusters them.  */
+
+static void
+find_duplicate (same_succ_t same_succ, basic_block bb1, basic_block bb2)
+{
+  gimple_stmt_iterator gsi1 = gsi_last_nondebug_bb (bb1);
+  gimple_stmt_iterator gsi2 = gsi_last_nondebug_bb (bb2);
+
+  gsi_advance_bw_nondebug_nonlocal (&gsi1);
+  gsi_advance_bw_nondebug_nonlocal (&gsi2);
+
+  while (!gsi_end_p (gsi1) && !gsi_end_p (gsi2))
+    {
+      if (!gimple_equal_p (same_succ, gsi_stmt (gsi1), gsi_stmt (gsi2)))
+	return;
+
+      gsi_prev_nondebug (&gsi1);
+      gsi_prev_nondebug (&gsi2);
+      gsi_advance_bw_nondebug_nonlocal (&gsi1);
+      gsi_advance_bw_nondebug_nonlocal (&gsi2);
+    }
+
+  if (!(gsi_end_p (gsi1) && gsi_end_p (gsi2)))
+    return;
+
+  if (dump_file)
+    fprintf (dump_file, "find_duplicates: <bb %d> duplicate of <bb %d>\n",
+	     bb1->index, bb2->index);
+
+  set_cluster (bb1, bb2);
+}
+
+/* Returns whether for all phis in DEST the phi alternatives for E1 and
+   E2 are equal.  */
+
+static bool
+same_phi_alternatives_1 (basic_block dest, edge e1, edge e2)
+{
+  int n1 = e1->dest_idx, n2 = e2->dest_idx;
+  gimple_stmt_iterator gsi;
+
+  for (gsi = gsi_start_phis (dest); !gsi_end_p (gsi); gsi_next (&gsi))
+    {
+      gimple phi = gsi_stmt (gsi);
+      tree lhs = gimple_phi_result (phi);
+      tree val1 = gimple_phi_arg_def (phi, n1);
+      tree val2 = gimple_phi_arg_def (phi, n2);
+
+      if (!is_gimple_reg (lhs))
+	continue;
+
+      if (operand_equal_for_phi_arg_p (val1, val2))
+        continue;
+      if (gvn_uses_equal (val1, val2))
+	continue;
+
+      return false;
+    }
+
+  return true;
+}
+
+/* Returns whether for all successors of BB1 and BB2 (members of SAME_SUCC), the
+   phi alternatives for BB1 and BB2 are equal.  */
+
+static bool
+same_phi_alternatives (same_succ_t same_succ, basic_block bb1, basic_block bb2)
+{
+  unsigned int s;
+  bitmap_iterator bs;
+  edge e1, e2;
+  basic_block succ;
+
+  EXECUTE_IF_SET_IN_BITMAP (same_succ->succs, 0, s, bs)
+    {
+      succ = BASIC_BLOCK (s);
+      e1 = find_edge (bb1, succ);
+      e2 = find_edge (bb2, succ);
+      if (e1->flags & EDGE_COMPLEX
+	  || e2->flags & EDGE_COMPLEX)
+	return false;
+
+      /* For all phis in bb, the phi alternatives for e1 and e2 need to have
+	 the same value.  */
+      if (!same_phi_alternatives_1 (succ, e1, e2))
+	return false;
+    }
+
+  return true;
+}
+
+/* Return true if BB has non-vop phis.  */
+
+static bool
+bb_has_non_vop_phi (basic_block bb)
+{
+  gimple_seq phis = phi_nodes (bb);
+  gimple phi;
+
+  if (phis == NULL)
+    return false;
+
+  if (!gimple_seq_singleton_p (phis))
+    return true;
+
+  phi = gimple_seq_first_stmt (phis);
+  return is_gimple_reg (gimple_phi_result (phi));
+}
+
+/* Returns true if redirecting the incoming edges of FROM to TO maintains the
+   invariant that uses in FROM are dominates by their defs.  */
+
+static bool
+deps_ok_for_redirect_from_bb_to_bb (basic_block from, basic_block to)
+{
+  basic_block cd, dep_bb = BB_DEP_BB (to);
+  edge_iterator ei;
+  edge e;
+  bitmap from_preds = BITMAP_ALLOC (NULL);
+
+  if (dep_bb == NULL)
+    return true;
+
+  FOR_EACH_EDGE (e, ei, from->preds)
+    bitmap_set_bit (from_preds, e->src->index);
+  cd = nearest_common_dominator_for_set (CDI_DOMINATORS, from_preds);
+  BITMAP_FREE (from_preds);
+
+  return dominated_by_p (CDI_DOMINATORS, dep_bb, cd);
+}
+
+/* Returns true if replacing BB1 (or its replacement bb) by BB2 (or its
+   replacement bb) and vice versa maintains the invariant that uses in the
+   replacement are dominates by their defs.  */
+
+static bool
+deps_ok_for_redirect (basic_block bb1, basic_block bb2)
+{
+  if (BB_CLUSTER (bb1) != NULL)
+    bb1 = BB_CLUSTER (bb1)->rep_bb;
+
+  if (BB_CLUSTER (bb2) != NULL)
+    bb2 = BB_CLUSTER (bb2)->rep_bb;
+
+  return (deps_ok_for_redirect_from_bb_to_bb (bb1, bb2)
+	  && deps_ok_for_redirect_from_bb_to_bb (bb2, bb1));
+}
+
+/* Within SAME_SUCC->bbs, find clusters of bbs which can be merged.  */
+
+static void
+find_clusters_1 (same_succ_t same_succ)
+{
+  basic_block bb1, bb2;
+  unsigned int i, j;
+  bitmap_iterator bi, bj;
+  int nr_comparisons;
+  int max_comparisons = PARAM_VALUE (PARAM_MAX_TAIL_MERGE_COMPARISONS);
+
+  EXECUTE_IF_SET_IN_BITMAP (same_succ->bbs, 0, i, bi)
+    {
+      bb1 = BASIC_BLOCK (i);
+
+      /* TODO: handle blocks with phi-nodes.  We'll have to find corresponding
+	 phi-nodes in bb1 and bb2, with the same alternatives for the same
+	 preds.  */
+      if (bb_has_non_vop_phi (bb1))
+	continue;
+
+      nr_comparisons = 0;
+      EXECUTE_IF_SET_IN_BITMAP (same_succ->bbs, i + 1, j, bj)
+	{
+	  bb2 = BASIC_BLOCK (j);
+
+	  if (bb_has_non_vop_phi (bb2))
+	    continue;
+
+	  if (BB_CLUSTER (bb1) != NULL && BB_CLUSTER (bb1) == BB_CLUSTER (bb2))
+	    continue;
+
+	  /* Limit quadratic behaviour.  */
+	  nr_comparisons++;
+	  if (nr_comparisons > max_comparisons)
+	    break;
+
+	  /* This is a conservative dependency check.  We could test more
+	     precise for allowed replacement direction.  */
+	  if (!deps_ok_for_redirect (bb1, bb2))
+	    continue;
+
+	  if (!(same_phi_alternatives (same_succ, bb1, bb2)))
+	    continue;
+
+	  find_duplicate (same_succ, bb1, bb2);
+        }
+    }
+}
+
+/* Find clusters of bbs which can be merged.  */
+
+static void
+find_clusters (void)
+{
+  same_succ_t same;
+
+  while (!VEC_empty (same_succ_t, worklist))
+    {
+      same = VEC_pop (same_succ_t, worklist);
+      same->in_worklist = false;
+      if (dump_file && (dump_flags & TDF_DETAILS))
+	{
+	  fprintf (dump_file, "processing worklist entry\n");
+	  same_succ_print (dump_file, same);
+	}
+      find_clusters_1 (same);
+    }
+}
+
+/* Create or update a vop phi in BB2.  Use VUSE1 arguments for all the
+   REDIRECTED_EDGES, or if VUSE1 is NULL_TREE, use BB_VOP_AT_EXIT.  If a new
+   phis is created, use the phi instead of VUSE2 in BB2.  */
+
+static void
+update_vuses (tree vuse1, tree vuse2, basic_block bb2,
+              VEC (edge,heap) *redirected_edges)
+{
+  gimple stmt, phi = NULL;
+  tree lhs = NULL_TREE, arg;
+  unsigned int i;
+  gimple def_stmt2;
+  imm_use_iterator iter;
+  use_operand_p use_p;
+  edge_iterator ei;
+  edge e;
+
+  def_stmt2 = SSA_NAME_DEF_STMT (vuse2);
+
+  if (gimple_bb (def_stmt2) == bb2)
+    /* Update existing phi.  */
+    phi = def_stmt2;
+  else
+    {
+      /* No need to create a phi with 2 equal arguments.  */
+      if (vuse1 == vuse2)
+	return;
+
+      /* Create a phi.  */
+      lhs = make_ssa_name (SSA_NAME_VAR (vuse2), NULL);
+      VN_INFO_GET (lhs);
+      phi = create_phi_node (lhs, bb2);
+      SSA_NAME_DEF_STMT (lhs) = phi;
+
+      /* Set default argument vuse2 for all preds.  */
+      FOR_EACH_EDGE (e, ei, bb2->preds)
+	add_phi_arg (phi, vuse2, e, UNKNOWN_LOCATION);
+    }
+
+  /* Update phi.  */
+  for (i = 0; i < EDGE_COUNT (redirected_edges); ++i)
+    {
+      e = VEC_index (edge, redirected_edges, i);
+      if (vuse1 != NULL_TREE)
+	arg = vuse1;
+      else
+	arg = BB_VOP_AT_EXIT (e->src);
+      add_phi_arg (phi, arg, e, UNKNOWN_LOCATION);
+    }
+
+  /* Return if we updated an existing phi.  */
+  if (gimple_bb (def_stmt2) == bb2)
+    return;
+
+  /* Replace relevant uses of vuse2 with the newly created phi.  */
+  FOR_EACH_IMM_USE_STMT (stmt, iter, vuse2)
+    {
+      if (stmt == phi)
+	continue;
+      if (gimple_code (stmt) != GIMPLE_PHI)
+	if (gimple_bb (stmt) != bb2)
+	  continue;
+
+      FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
+	{
+	  if (gimple_code (stmt) == GIMPLE_PHI)
+	    {
+	      unsigned int pred_index = PHI_ARG_INDEX_FROM_USE (use_p);
+	      basic_block pred = EDGE_PRED (gimple_bb (stmt), pred_index)->src;
+	      if (pred !=  bb2)
+		continue;
+	    }
+	  SET_USE (use_p, lhs);
+	  update_stmt (stmt);
+	}
+    }
+}
+
+/* Returns the vop phi of BB, if any.  */
+
+static gimple
+vop_phi (basic_block bb)
+{
+  gimple stmt;
+  gimple_stmt_iterator gsi;
+  for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
+    {
+      stmt = gsi_stmt (gsi);
+      if (is_gimple_reg (gimple_phi_result (stmt)))
+	continue;
+      return stmt;
+    }
+  return NULL;
+}
+
+/* Returns the vop state at the entry of BB, if found in BB or a successor
+   bb.  */
+
+static tree
+vop_at_entry (basic_block bb)
+{
+  gimple bb_phi, succ_phi;
+  gimple_stmt_iterator gsi;
+  gimple stmt;
+  tree vuse, vdef;
+  basic_block succ;
+
+  bb_phi = vop_phi (bb);
+  if (bb_phi != NULL)
+    return gimple_phi_result (bb_phi);
+
+  for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
+    {
+      stmt = gsi_stmt (gsi);
+      vuse = gimple_vuse (stmt);
+      vdef = gimple_vdef (stmt);
+      if (vuse != NULL_TREE)
+	return vuse;
+      if (vdef != NULL_TREE)
+	return NULL_TREE;
+    }
+
+  if (EDGE_COUNT (bb->succs) == 0)
+    return NULL_TREE;
+
+  succ = EDGE_SUCC (bb, 0)->dest;
+  succ_phi = vop_phi (succ);
+  return (succ_phi != NULL
+	  ? PHI_ARG_DEF_FROM_EDGE (succ_phi, find_edge (bb, succ))
+	  : NULL_TREE);
+}
+
+/* Redirect all edges from BB1 to BB2, marks BB1 for removal, and if
+   UPDATE_VOPS, inserts vop phis.  */
+
+static void
+replace_block_by (basic_block bb1, basic_block bb2, bool update_vops)
+{
+  edge pred_edge;
+  unsigned int i;
+  tree phi_vuse1 = NULL_TREE, phi_vuse2 = NULL_TREE, arg;
+  VEC (edge,heap) *redirected_edges = NULL;
+  edge e;
+  edge_iterator ei;
+
+  if (update_vops)
+    {
+      /* Find the vops at entry of bb1 and bb2.  */
+      phi_vuse1 = vop_at_entry (bb1);
+      phi_vuse2 = vop_at_entry (bb2);
+
+      /* If one of the 2 not found, it means there's no need to update.  */
+      update_vops = phi_vuse1 != NULL_TREE && phi_vuse2 != NULL_TREE;
+    }
+
+  if (update_vops && gimple_bb (SSA_NAME_DEF_STMT (phi_vuse1)) == bb1)
+    {
+      /* If the vop at entry of bb1 is a phi, save the phi alternatives in
+	 BB_VOP_AT_EXIT, before we lose that information by redirecting the
+	 edges.  */
+      FOR_EACH_EDGE (e, ei, bb1->preds)
+	{
+	  arg = PHI_ARG_DEF_FROM_EDGE (SSA_NAME_DEF_STMT (phi_vuse1), e);
+	  BB_VOP_AT_EXIT (e->src) = arg;
+	}
+      phi_vuse1 = NULL;
+    }
+
+  /* Mark the basic block for later deletion.  */
+  delete_basic_block_same_succ (bb1);
+
+  if (update_vops)
+    redirected_edges = VEC_alloc (edge, heap, 10);
+
+  /* Redirect the incoming edges of bb1 to bb2.  */
+  for (i = EDGE_COUNT (bb1->preds); i > 0 ; --i)
+    {
+      pred_edge = EDGE_PRED (bb1, i - 1);
+      pred_edge = redirect_edge_and_branch (pred_edge, bb2);
+      gcc_assert (pred_edge != NULL);
+      if (update_vops)
+	VEC_safe_push (edge, heap, redirected_edges, pred_edge);
+    }
+
+  /* Update the vops.  */
+  if (update_vops)
+    {
+      update_vuses (phi_vuse1, phi_vuse2, bb2, redirected_edges);
+      VEC_free (edge, heap, redirected_edges);
+    }
+}
+
+/* Bbs for which update_debug_stmt need to be called.  */
+
+static bitmap update_bbs;
+
+/* For each cluster in all_clusters, merge all cluster->bbs.  Returns
+   number of bbs removed.  Insert vop phis if UPDATE_VOPS.  */
+
+static int
+apply_clusters (bool update_vops)
+{
+  basic_block bb1, bb2;
+  bb_cluster_t c;
+  unsigned int i, j;
+  bitmap_iterator bj;
+  int nr_bbs_removed = 0;
+
+  for (i = 0; i < VEC_length (bb_cluster_t, all_clusters); ++i)
+    {
+      c = VEC_index (bb_cluster_t, all_clusters, i);
+      if (c == NULL)
+	continue;
+
+      bb2 = c->rep_bb;
+      bitmap_set_bit (update_bbs, bb2->index);
+
+      bitmap_clear_bit (c->bbs, bb2->index);
+      EXECUTE_IF_SET_IN_BITMAP (c->bbs, 0, j, bj)
+	{
+	  bb1 = BASIC_BLOCK (j);
+	  bitmap_clear_bit (update_bbs, bb1->index);
+
+	  replace_block_by (bb1, bb2, update_vops);
+	  nr_bbs_removed++;
+	}
+    }
+
+  return nr_bbs_removed;
+}
+
+/* Resets debug statement STMT if it has uses that are not dominated by their
+   defs.  */
+
+static void
+update_debug_stmt (gimple stmt)
+{
+  use_operand_p use_p;
+  ssa_op_iter oi;
+  basic_block bbdef, bbuse;
+  gimple def_stmt;
+  tree name;
+
+  if (!gimple_debug_bind_p (stmt))
+    return;
+
+  bbuse = gimple_bb (stmt);
+  FOR_EACH_PHI_OR_STMT_USE (use_p, stmt, oi, SSA_OP_USE)
+    {
+      name = USE_FROM_PTR (use_p);
+      gcc_assert (TREE_CODE (name) == SSA_NAME);
+
+      def_stmt = SSA_NAME_DEF_STMT (name);
+      gcc_assert (def_stmt != NULL);
+
+      bbdef = gimple_bb (def_stmt);
+      if (bbdef == NULL || bbuse == bbdef
+	  || dominated_by_p (CDI_DOMINATORS, bbuse, bbdef))
+	continue;
+
+      gimple_debug_bind_reset_value (stmt);
+      update_stmt (stmt);
+    }
+}
+
+/* Resets all debug statements that have uses that are not
+   dominated by their defs.  */
+
+static void
+update_debug_stmts (void)
+{
+  basic_block bb;
+  bitmap_iterator bi;
+  unsigned int i;
+
+  if (!MAY_HAVE_DEBUG_STMTS)
+    return;
+
+  EXECUTE_IF_SET_IN_BITMAP (update_bbs, 0, i, bi)
+    {
+      gimple stmt;
+      gimple_stmt_iterator gsi;
+
+      bb = BASIC_BLOCK (i);
+      for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
+	{
+	  stmt = gsi_stmt (gsi);
+	  if (!is_gimple_debug (stmt))
+	    continue;
+	  update_debug_stmt (stmt);
+	}
+    }
+}
+
+/* Runs tail merge optimization.  */
+
+unsigned int
+tail_merge_optimize (unsigned int todo)
+{
+  int nr_bbs_removed_total = 0;
+  int nr_bbs_removed;
+  bool loop_entered = false;
+  int iteration_nr = 0;
+  bool update_vops = !symbol_marked_for_renaming (gimple_vop (cfun));
+
+  if (!flag_tree_tail_merge)
+    return 0;
+
+  timevar_push (TV_TREE_TAIL_MERGE);
+
+  calculate_dominance_info (CDI_DOMINATORS);
+  init_worklist ();
+
+  while (!VEC_empty (same_succ_t, worklist))
+    {
+      if (!loop_entered)
+	{
+	  loop_entered = true;
+	  alloc_cluster_vectors ();
+	  update_bbs = BITMAP_ALLOC (NULL);
+	}
+      else
+	reset_cluster_vectors ();
+
+      iteration_nr++;
+      if (dump_file && (dump_flags & TDF_DETAILS))
+	fprintf (dump_file, "worklist iteration #%d\n", iteration_nr);
+
+      find_clusters ();
+      gcc_assert (VEC_empty (same_succ_t, worklist));
+      if (VEC_empty (bb_cluster_t, all_clusters))
+	break;
+
+      nr_bbs_removed = apply_clusters (update_vops);
+      nr_bbs_removed_total += nr_bbs_removed;
+      if (nr_bbs_removed == 0)
+	break;
+
+      free_dominance_info (CDI_DOMINATORS);
+      purge_bbs ();
+
+      calculate_dominance_info (CDI_DOMINATORS);
+      update_worklist ();
+    }
+
+  if (dump_file && (dump_flags & TDF_DETAILS))
+    fprintf (dump_file, "htab collision / search: %f\n",
+	     htab_collisions (same_succ_htab));
+
+  if (nr_bbs_removed_total > 0)
+    {
+      update_debug_stmts ();
+
+      if (dump_file && (dump_flags & TDF_DETAILS))
+	{
+	  fprintf (dump_file, "Before TODOs.\n");
+	  dump_function_to_file (current_function_decl, dump_file, dump_flags);
+	}
+
+      todo |= (TODO_verify_ssa | TODO_verify_stmts | TODO_verify_flow
+	       | TODO_dump_func);
+    }
+
+  delete_worklist ();
+  if (loop_entered)
+    {
+      delete_cluster_vectors ();
+      BITMAP_FREE (update_bbs);
+    }
+
+  timevar_pop (TV_TREE_TAIL_MERGE);
+
+  return todo;
+}
Index: gcc/tree-pass.h
===================================================================
--- gcc/tree-pass.h	(revision 176554)
+++ gcc/tree-pass.h	(working copy)
@@ -401,6 +401,7 @@  extern struct gimple_opt_pass pass_call_
 extern struct gimple_opt_pass pass_merge_phi;
 extern struct gimple_opt_pass pass_split_crit_edges;
 extern struct gimple_opt_pass pass_pre;
+extern unsigned int tail_merge_optimize (unsigned int);
 extern struct gimple_opt_pass pass_profile;
 extern struct gimple_opt_pass pass_strip_predict_hints;
 extern struct gimple_opt_pass pass_lower_complex_O0;
Index: gcc/tree-ssa-sccvn.c
===================================================================
--- gcc/tree-ssa-sccvn.c	(revision 176554)
+++ gcc/tree-ssa-sccvn.c	(working copy)
@@ -2896,19 +2896,6 @@  simplify_unary_expression (gimple stmt)
   return NULL_TREE;
 }
 
-/* Valueize NAME if it is an SSA name, otherwise just return it.  */
-
-static inline tree
-vn_valueize (tree name)
-{
-  if (TREE_CODE (name) == SSA_NAME)
-    {
-      tree tem = SSA_VAL (name);
-      return tem == VN_TOP ? name : tem;
-    }
-  return name;
-}
-
 /* Try to simplify RHS using equivalences and constant folding.  */
 
 static tree
Index: gcc/tree-ssa-sccvn.h
===================================================================
--- gcc/tree-ssa-sccvn.h	(revision 176554)
+++ gcc/tree-ssa-sccvn.h	(working copy)
@@ -209,4 +209,18 @@  unsigned int get_constant_value_id (tree
 unsigned int get_or_alloc_constant_value_id (tree);
 bool value_id_constant_p (unsigned int);
 tree fully_constant_vn_reference_p (vn_reference_t);
+
+/* Valueize NAME if it is an SSA name, otherwise just return it.  */
+
+static inline tree
+vn_valueize (tree name)
+{
+  if (TREE_CODE (name) == SSA_NAME)
+    {
+      tree tem = VN_INFO (name)->valnum;
+      return tem == VN_TOP ? name : tem;
+    }
+  return name;
+}
+
 #endif /* TREE_SSA_SCCVN_H  */
Index: gcc/opts.c
===================================================================
--- gcc/opts.c	(revision 176554)
+++ gcc/opts.c	(working copy)
@@ -484,6 +484,7 @@  static const struct default_options defa
     { OPT_LEVELS_2_PLUS, OPT_falign_jumps, NULL, 1 },
     { OPT_LEVELS_2_PLUS, OPT_falign_labels, NULL, 1 },
     { OPT_LEVELS_2_PLUS, OPT_falign_functions, NULL, 1 },
+    { OPT_LEVELS_2_PLUS, OPT_ftree_tail_merge, NULL, 1 },
 
     /* -O3 optimizations.  */
     { OPT_LEVELS_3_PLUS, OPT_ftree_loop_distribute_patterns, NULL, 1 },
Index: gcc/timevar.def
===================================================================
--- gcc/timevar.def	(revision 176554)
+++ gcc/timevar.def	(working copy)
@@ -127,6 +127,7 @@  DEFTIMEVAR (TV_TREE_GIMPLIFY	     , "tre
 DEFTIMEVAR (TV_TREE_EH		     , "tree eh")
 DEFTIMEVAR (TV_TREE_CFG		     , "tree CFG construction")
 DEFTIMEVAR (TV_TREE_CLEANUP_CFG	     , "tree CFG cleanup")
+DEFTIMEVAR (TV_TREE_TAIL_MERGE       , "tree tail merge")
 DEFTIMEVAR (TV_TREE_VRP              , "tree VRP")
 DEFTIMEVAR (TV_TREE_COPY_PROP        , "tree copy propagation")
 DEFTIMEVAR (TV_FIND_REFERENCED_VARS  , "tree find ref. vars")
Index: gcc/tree-ssa-pre.c
===================================================================
--- gcc/tree-ssa-pre.c	(revision 176554)
+++ gcc/tree-ssa-pre.c	(working copy)
@@ -4926,7 +4926,6 @@  execute_pre (bool do_fre)
   statistics_counter_event (cfun, "Constified", pre_stats.constified);
 
   clear_expression_ids ();
-  free_scc_vn ();
   if (!do_fre)
     {
       remove_dead_inserted_code ();
@@ -4936,6 +4935,17 @@  execute_pre (bool do_fre)
   scev_finalize ();
   fini_pre (do_fre);
 
+  if (!do_fre)
+    /* TODO: tail_merge_optimize may merge all predecessors of a block, in which
+       case we can merge the block with the remaining predecessor of the block.
+       It should either:
+       - call merge_blocks after each tail merge iteration
+       - call merge_blocks after all tail merge iterations
+       - mark TODO_cleanup_cfg when necessary
+       - share the cfg cleanup with fini_pre.  */
+    todo |= tail_merge_optimize (todo);
+  free_scc_vn ();
+
   return todo;
 }
 
Index: gcc/common.opt
===================================================================
--- gcc/common.opt	(revision 176554)
+++ gcc/common.opt	(working copy)
@@ -1937,6 +1937,10 @@  ftree-dominator-opts
 Common Report Var(flag_tree_dom) Optimization
 Enable dominator optimizations
 
+ftree-tail-merge
+Common Report Var(flag_tree_tail_merge) Optimization
+Enable tail merging on trees
+
 ftree-dse
 Common Report Var(flag_tree_dse) Optimization
 Enable dead store elimination
Index: gcc/Makefile.in
===================================================================
--- gcc/Makefile.in	(revision 176554)
+++ gcc/Makefile.in	(working copy)
@@ -1465,6 +1465,7 @@  OBJS = \
 	tree-ssa-sccvn.o \
 	tree-ssa-sink.o \
 	tree-ssa-structalias.o \
+	tree-ssa-tail-merge.o \
 	tree-ssa-ter.o \
 	tree-ssa-threadedge.o \
 	tree-ssa-threadupdate.o \
@@ -2373,6 +2374,13 @@  stor-layout.o : stor-layout.c $(CONFIG_H
    $(TREE_H) $(PARAMS_H) $(FLAGS_H) $(FUNCTION_H) $(EXPR_H) output.h $(RTL_H) \
    $(GGC_H) $(TM_P_H) $(TARGET_H) langhooks.h $(REGS_H) gt-stor-layout.h \
    $(DIAGNOSTIC_CORE_H) $(CGRAPH_H) $(TREE_INLINE_H) $(TREE_DUMP_H) $(GIMPLE_H)
+tree-ssa-tail-merge.o: tree-ssa-tail-merge.c \
+   $(SYSTEM_H) $(CONFIG_H) coretypes.h $(TM_H) $(BITMAP_H) \
+   $(FLAGS_H) $(TM_P_H) $(BASIC_BLOCK_H) output.h \
+   $(TREE_H) $(TREE_FLOW_H) $(TREE_INLINE_H) \
+   $(GIMPLE_H) $(FUNCTION_H) \
+   $(TIMEVAR_H) tree-ssa-sccvn.h \
+   $(CGRAPH_H) gimple-pretty-print.h tree-pretty-print.h $(PARAMS_H)
 tree-ssa-structalias.o: tree-ssa-structalias.c \
    $(SYSTEM_H) $(CONFIG_H) coretypes.h $(TM_H) $(GGC_H) $(OBSTACK_H) $(BITMAP_H) \
    $(FLAGS_H) $(TM_P_H) $(BASIC_BLOCK_H) output.h \
Index: gcc/params.def
===================================================================
--- gcc/params.def	(revision 176554)
+++ gcc/params.def	(working copy)
@@ -908,6 +908,11 @@  DEFPARAM (PARAM_CASE_VALUES_THRESHOLD,
 	  "if 0, use the default for the machine",
           0, 0, 0)
 
+DEFPARAM (PARAM_MAX_TAIL_MERGE_COMPARISONS,
+          "max-tail-merge-comparisons",
+          "Maximum amount of similar bbs to compare a bb with",
+          10, 0, 0)
+
 
 /*
 Local variables:
Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi	(revision 176554)
+++ gcc/doc/invoke.texi	(working copy)
@@ -404,7 +404,7 @@  Objective-C and Objective-C++ Dialects}.
 -ftree-phiprop -ftree-loop-distribution -ftree-loop-distribute-patterns @gol
 -ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize @gol
 -ftree-parallelize-loops=@var{n} -ftree-pre -ftree-pta -ftree-reassoc @gol
--ftree-sink -ftree-sra -ftree-switch-conversion @gol
+-ftree-sink -ftree-sra -ftree-switch-conversion -ftree-tail-merge @gol
 -ftree-ter -ftree-vect-loop-version -ftree-vectorize -ftree-vrp @gol
 -funit-at-a-time -funroll-all-loops -funroll-loops @gol
 -funsafe-loop-optimizations -funsafe-math-optimizations -funswitch-loops @gol
@@ -6096,7 +6096,7 @@  also turns on the following optimization
 -fsched-interblock  -fsched-spec @gol
 -fschedule-insns  -fschedule-insns2 @gol
 -fstrict-aliasing -fstrict-overflow @gol
--ftree-switch-conversion @gol
+-ftree-switch-conversion -ftree-tail-merge @gol
 -ftree-pre @gol
 -ftree-vrp}
 
@@ -6979,6 +6979,11 @@  Perform conversion of simple initializat
 initializations from a scalar array.  This flag is enabled by default
 at @option{-O2} and higher.
 
+@item -ftree-tail-merge
+Merges identical blocks with same successors.  This flag is enabled by default
+at @option{-O2} and higher.  The run time of this pass can be limited using
+@option{max-tail-merge-comparisons} parameter.
+
 @item -ftree-dce
 @opindex ftree-dce
 Perform dead code elimination (DCE) on trees.  This flag is enabled by
@@ -8546,6 +8551,10 @@  This is used to avoid quadratic behavior
 The value of 0 will avoid limiting the search, but may slow down compilation
 of huge functions.  The default value is 30.
 
+@item max-tail-merge-comparisons
+The maximum amount of similar bbs to compare a bb with.  This is used to
+avoid quadratic behaviour in tree tail merging.  The default value is 10.
+
 @item max-unrolled-insns
 The maximum number of instructions that a loop should have if that loop
 is unrolled, and if the loop is unrolled, it determines how many times